Older veterans who suffer from posttraumatic stress disorder (PTSD) are almost twice as likely to develop Alzheimer's disease and other age-related dementias as veterans without PTSD, a study shows.

The study is among the first to link combat-related posttraumatic stress disorder to dementia later in life, but it is not clear if having PTSD increases the risk for late-life dementias or if recurring PTSD is an early symptom of dementia in older veterans, Deborah Barnes, PhD, of the University of California, San Francisco, tells WebMD.

"We can't say from a study like this one that PTSD causes dementia, " she says. "But if it does, one theory is that stress is to blame."

There is evidence that chronic stress may damage the hippocampus, which is the area of the brain that is critical for memory and learning.

Barnes and colleagues followed more than 180,000 mostly male older veterans for seven years, including just over 53,000 who had a diagnosis of PTSD. None had dementia in late 2000, but roughly 31,000 (17%) had been diagnosed with the degenerative memory and thinking disorder by late 2007.